The mitotic spindle is the cellular apparatus responsible for the accurate segregation of chromosomes during cell division. Missegregation of chromosomes during mitosis leads to aneuploidy, the condition of having an improper number of chromosomes. Aneuploidy is frequently observed in cancers and may be one of the inducing or promoting factors of cancer. Thus, a thorough understanding of the cellular mechanisms that control the mitotic spindle is important for understanding cancer cell development and physiology. Observation of mitotic spindle dynamics may reveal cellular pathology before other signs are apparent. This proposal seeks to develop the embryo of the African clawed frog Xenopus laevis as a model organism for the study of mitotic spindle pathology. The X. laevis system offers many distinct advantages over other systems, including the ability to observe cellular pathology in the context of a whole, living vertebrate organism. The development of cellular probes for mitotic spindle imaging coupled with customized image processing software will provide an invaluable tool for further mitotic spindle investigations. These tools will be applied to two distinct lines of investigation. First, depletion of Myosin-10, a motor protein with both actin and microtubule binding capacity, destabilizes the mitotic spindle and leads to multipolarity and abnormal mitoses. Myosin-10 may influence the cell cycle through interaction with cell cycle regulators Wee1 kinase and Greatwall kinase. These interactions will be investigated using in vitro assays with purified protein and in vivo observations of the mitotic spindle. Second, this proposal will test the hypothesis that mitotic abnormalities exist before frank tumors can be observed. Tumor formation will be induced by expression of an oncogene or dominant-negative tumor suppressor. Mitotic spindle dynamics will be observed and compared throughout tumor development, leading to a better understanding of mitotic pathology in tumor cells.